[PATCH v19 5/7] x86/crash: add x86 crash hotplug support
Sourabh Jain
sourabhjain at linux.ibm.com
Tue Mar 7 19:08:36 PST 2023
On 06/03/23 21:52, Eric DeVolder wrote:
> When CPU or memory is hot un/plugged, or off/onlined, the crash
> elfcorehdr, which describes the CPUs and memory in the system,
> must also be updated.
>
> The segment containing the elfcorehdr is identified at run-time
> in crash_core:crash_handle_hotplug_event(), which works for both
> the kexec_load() and kexec_file_load() syscalls. A new elfcorehdr
> is generated from the available CPUs and memory into a buffer,
> and then installed over the top of the existing elfcorehdr.
>
> In the patch 'kexec: exclude elfcorehdr from the segment digest'
> the need to update purgatory due to the change in elfcorehdr was
> eliminated. As a result, no changes to purgatory or boot_params
> (as the elfcorehdr= kernel command line parameter pointer
> remains unchanged and correct) are needed, just elfcorehdr.
>
> To accommodate a growing number of resources via hotplug, the
> elfcorehdr segment must be sufficiently large enough to accommodate
> changes, see the CRASH_MAX_MEMORY_RANGES description. This is used
> only on the kexec_file_load() syscall; for kexec_load() userspace
> will need to size the segment similarly.
>
> To accommodate kexec_load() syscall in the absence of
> kexec_file_load() syscall support, and with CONFIG_CRASH_HOTPLUG
> enabled, it is necessary to move prepare_elf_headers() and
> dependents outside of CONFIG_KEXEC_FILE.
>
> Signed-off-by: Eric DeVolder <eric.devolder at oracle.com>
> ---
> arch/x86/Kconfig | 13 ++++
> arch/x86/include/asm/kexec.h | 15 +++++
> arch/x86/kernel/crash.c | 119 ++++++++++++++++++++++++++++++++---
> 3 files changed, 140 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 3604074a878b..2ca5e19b8f19 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2119,6 +2119,19 @@ config CRASH_DUMP
> (CONFIG_RELOCATABLE=y).
> For more details see Documentation/admin-guide/kdump/kdump.rst
>
> +config CRASH_HOTPLUG
> + bool "Update the crash elfcorehdr on system configuration changes"
> + default n
How about enabling this by default?
- Sourabh
> + depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG)
> + help
> + Enable direct update to the crash elfcorehdr (which contains
> + the list of CPUs and memory regions to be dumped upon a crash)
> + in response to hot plug/unplug or online/offline of CPUs or
> + memory. This is a much more advanced approach than userspace
> + attempting that.
> +
> + If unsure, say Y.
> +
> config KEXEC_JUMP
> bool "kexec jump"
> depends on KEXEC && HIBERNATION
> diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
> index a3760ca796aa..1bc852ce347d 100644
> --- a/arch/x86/include/asm/kexec.h
> +++ b/arch/x86/include/asm/kexec.h
> @@ -212,6 +212,21 @@ typedef void crash_vmclear_fn(void);
> extern crash_vmclear_fn __rcu *crash_vmclear_loaded_vmcss;
> extern void kdump_nmi_shootdown_cpus(void);
>
> +#ifdef CONFIG_CRASH_HOTPLUG
> +void arch_crash_handle_hotplug_event(struct kimage *image);
> +#define arch_crash_handle_hotplug_event arch_crash_handle_hotplug_event
> +
> +#ifdef CONFIG_HOTPLUG_CPU
> +static inline int crash_hotplug_cpu_support(void) { return 1; }
> +#define crash_hotplug_cpu_support crash_hotplug_cpu_support
> +#endif
> +
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +static inline int crash_hotplug_memory_support(void) { return 1; }
> +#define crash_hotplug_memory_support crash_hotplug_memory_support
> +#endif
> +#endif
> +
> #endif /* __ASSEMBLY__ */
>
> #endif /* _ASM_X86_KEXEC_H */
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index 305514431f26..c9aaec9de775 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -42,6 +42,21 @@
> #include <asm/crash.h>
> #include <asm/cmdline.h>
>
> +/*
> + * For the kexec_file_load() syscall path, specify the maximum number of
> + * memory regions that the elfcorehdr buffer/segment can accommodate.
> + * These regions are obtained via walk_system_ram_res(); eg. the
> + * 'System RAM' entries in /proc/iomem.
> + * This value is combined with NR_CPUS_DEFAULT and multiplied by
> + * sizeof(Elf64_Phdr) to determine the final elfcorehdr memory buffer/
> + * segment size.
> + * The value 8192, for example, covers a (sparsely populated) 1TiB system
> + * consisting of 128MiB memblocks, while resulting in an elfcorehdr
> + * memory buffer/segment size under 1MiB. This represents a sane choice
> + * to accommodate both baremetal and virtual machine configurations.
> + */
> +#define CRASH_MAX_MEMORY_RANGES 8192
> +
> /* Used while preparing memory map entries for second kernel */
> struct crash_memmap_data {
> struct boot_params *params;
> @@ -173,8 +188,6 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
> crash_save_cpu(regs, safe_smp_processor_id());
> }
>
> -#ifdef CONFIG_KEXEC_FILE
> -
> static int get_nr_ram_ranges_callback(struct resource *res, void *arg)
> {
> unsigned int *nr_ranges = arg;
> @@ -246,7 +259,7 @@ static int prepare_elf64_ram_headers_callback(struct resource *res, void *arg)
>
> /* Prepare elf headers. Return addr and size */
> static int prepare_elf_headers(struct kimage *image, void **addr,
> - unsigned long *sz)
> + unsigned long *sz, unsigned long *nr_mem_ranges)
> {
> struct crash_mem *cmem;
> int ret;
> @@ -264,6 +277,9 @@ static int prepare_elf_headers(struct kimage *image, void **addr,
> if (ret)
> goto out;
>
> + /* Return the computed number of memory ranges, for hotplug usage */
> + *nr_mem_ranges = cmem->nr_ranges;
> +
> /* By default prepare 64bit headers */
> ret = crash_prepare_elf64_headers(cmem, IS_ENABLED(CONFIG_X86_64), addr, sz);
>
> @@ -272,6 +288,7 @@ static int prepare_elf_headers(struct kimage *image, void **addr,
> return ret;
> }
>
> +#ifdef CONFIG_KEXEC_FILE
> static int add_e820_entry(struct boot_params *params, struct e820_entry *entry)
> {
> unsigned int nr_e820_entries;
> @@ -386,18 +403,42 @@ int crash_setup_memmap_entries(struct kimage *image, struct boot_params *params)
> int crash_load_segments(struct kimage *image)
> {
> int ret;
> + unsigned long pnum = 0;
> struct kexec_buf kbuf = { .image = image, .buf_min = 0,
> .buf_max = ULONG_MAX, .top_down = false };
>
> /* Prepare elf headers and add a segment */
> - ret = prepare_elf_headers(image, &kbuf.buffer, &kbuf.bufsz);
> + ret = prepare_elf_headers(image, &kbuf.buffer, &kbuf.bufsz, &pnum);
> if (ret)
> return ret;
>
> - image->elf_headers = kbuf.buffer;
> - image->elf_headers_sz = kbuf.bufsz;
> + image->elf_headers = kbuf.buffer;
> + image->elf_headers_sz = kbuf.bufsz;
> + kbuf.memsz = kbuf.bufsz;
> +
> +#ifdef CONFIG_CRASH_HOTPLUG
> + /*
> + * Ensure the elfcorehdr segment large enough for hotplug changes.
> + * Account for VMCOREINFO and kernel_map and maximum CPUs.
> + */
> + if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
> + pnum = 2 + CONFIG_NR_CPUS_DEFAULT + CRASH_MAX_MEMORY_RANGES;
> + else
> + pnum += 2 + CONFIG_NR_CPUS_DEFAULT;
> +
> + if (pnum < (unsigned long)PN_XNUM) {
> + kbuf.memsz = pnum * sizeof(Elf64_Phdr);
> + kbuf.memsz += sizeof(Elf64_Ehdr);
> +
> + image->elfcorehdr_index = image->nr_segments;
> +
> + /* Mark as usable to crash kernel, else crash kernel fails on boot */
> + image->elf_headers_sz = kbuf.memsz;
> + } else {
> + pr_err("number of Phdrs %lu exceeds max\n", pnum);
> + }
> +#endif
>
> - kbuf.memsz = kbuf.bufsz;
> kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
> kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
> ret = kexec_add_buffer(&kbuf);
> @@ -410,3 +451,67 @@ int crash_load_segments(struct kimage *image)
> return ret;
> }
> #endif /* CONFIG_KEXEC_FILE */
> +
> +#ifdef CONFIG_CRASH_HOTPLUG
> +
> +#undef pr_fmt
> +#define pr_fmt(fmt) "crash hp: " fmt
> +
> +/**
> + * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
> + * @image: the active struct kimage
> + *
> + * The new elfcorehdr is prepared in a kernel buffer, and then it is
> + * written on top of the existing/old elfcorehdr.
> + */
> +void arch_crash_handle_hotplug_event(struct kimage *image)
> +{
> + void *elfbuf = NULL, *old_elfcorehdr;
> + unsigned long nr_mem_ranges;
> + unsigned long mem, memsz;
> + unsigned long elfsz = 0;
> +
> + /*
> + * Create the new elfcorehdr reflecting the changes to CPU and/or
> + * memory resources.
> + */
> + if (prepare_elf_headers(image, &elfbuf, &elfsz, &nr_mem_ranges)) {
> + pr_err("unable to prepare elfcore headers");
> + goto out;
> + }
> +
> + /*
> + * Obtain address and size of the elfcorehdr segment, and
> + * check it against the new elfcorehdr buffer.
> + */
> + mem = image->segment[image->elfcorehdr_index].mem;
> + memsz = image->segment[image->elfcorehdr_index].memsz;
> + if (elfsz > memsz) {
> + pr_err("update elfcorehdr elfsz %lu > memsz %lu",
> + elfsz, memsz);
> + goto out;
> + }
> +
> + /*
> + * Copy new elfcorehdr over the old elfcorehdr at destination.
> + */
> + old_elfcorehdr = kmap_local_page(pfn_to_page(mem >> PAGE_SHIFT));
> + if (!old_elfcorehdr) {
> + pr_err("updating elfcorehdr failed\n");
> + goto out;
> + }
> +
> + /*
> + * Temporarily invalidate the crash image while the
> + * elfcorehdr is updated.
> + */
> + xchg(&kexec_crash_image, NULL);
> + memcpy_flushcache(old_elfcorehdr, elfbuf, elfsz);
> + xchg(&kexec_crash_image, image);
> + kunmap_local(old_elfcorehdr);
> + pr_debug("updated elfcorehdr\n");
> +
> +out:
> + vfree(elfbuf);
> +}
> +#endif
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